Non-Equilibrium Method for the Non-Invasive Estimation of Arterial PCO2

Background: Under conditions of pulmonary ventilation/perfusion mismatch, end-tidal PCO2 (ETPCO2) may severely underestimate arterial PCO2 (PaCO2). Objective: We aimed to develop a method for the accurate non-invasive estimation of PaCO2 from ETPCO2. Methods/Patients: The fact that ETPCO2 is a mixture of PCO2 from both the ventilated and perfused alveoli, and the ventilated but unperfused ones (‘alveolar deadspace’), was brought into mathematical terms. Using the model, by inspiring two gases with different CO2 content and determining the corresponding ETPCO2 values, the PCO2 of the ventilated and perfused alveoli (truePCO2) may be calculated as an estimate of PaCO2. The model was applied on 12 ventilated patients aged 53 to 78 yrs. Estimates of PaCO2 were compared to the results of invasive determination. Results: Conventional ETPCO2 and PaCO2 differed on average by 22 percent (95% confidence interval, 18 to 26). Depending on the difference between the two inspiratory PCO2 levels used, the unsigned error of the model-based estimate typically was 5 percent (95% confidence interval, 3.5 to 7) or better. Conclusion: We provide a non-invasive method for the accurate estimation of PaCO2 and suggest its implementation into ventilators for the close monitoring of pulmonary treatment response.